The present invention relates to a device and a method for thermally pre-stressing elongated objects made from glass, especially for pre-stressing glass tubes, ampoules, syringes, phials or carpules.
For increasing the strength of glass, two methods have been known. One of them is known as chemical pre-stressing, the other one as thermal pre-stressing.
In chemical pre-stressing of glass, pressure pre-stressing is achieved at the glass surface by varying the composition of the glass surface, compared with the interior of the glass. This is effected as a rule by an ion exchange process. Chemical pre-stressing of glass actually results in a relatively high increase of strength, but is very time-consuming.
In thermal pre-stressing of glass, used especially for flat glass or curved glass, the finished cut plates are conveyed into a device, in suspended or flat condition, where their surface is heated up quickly to a temperature of approximately 150 degrees above transformation temperature. Immediately thereafter, cold air is blown against the glass by a nozzle system adapted to the glass shape. As a result of the quick cooling-down process, the glass surface is frozen in an expanded-grating condition, while the interior of the glass has time to cool down slowly and to contract more effectively. Given the fact that the surface and the interior of the glass form a single unit, the result is a pressure pre-stress in the surface layer and a tension pre-stress in the interior of the glass.
In thermal pre-stressing different cooling agents can be used, such as air, oil or water. It is understood that liquids such as oil or water permit clearly higher cooling-down speeds and, thus, higher increases in strength, than to be reached with air, for example. Further, it is known that large glass elements of considerable thickness can be thermally pre-stressed relatively easily as the great thickness provides the possibility to realize temperature differences between the interior and the surface with the aim to achieve differences in stress. On the other hand, glass objects having a small thickness and a relative low coefficient of thermal expansion normally can be pre-stressed using gaseous agents only with difficulty as due to the small thickness sufficiently high temperature differences can be reached only with difficulty and only small stresses can be achieved by cooling as a result of the small coefficients of thermal expansion. As a rule, one therefore uses oil for thin objects because this permits clearly higher temperature differences to be achieved. In many cases, however, the use of oil is undesirable because oil leads to contaminations so that in the pharmaceutical industry, for example, the use of oil is excluded.
DE 645 699 describes a device and a method for thermally pre-stressing glass tubes, wherein the glass tube to be pre-stressed is suspended in vertical position and is fully enclosed by a jacket through which cooling air is blown against the outer surface of the tube from a plurality of nozzles, the cooling air being then exhausted through openings immediately neighboring such nozzles.
Although the prior device and the prior method are suited for thermally hardening glass tubes of bigger diameters and greater thickness, they are not suited for thermally pre-stressing glass tubes having smaller diameters, for example in the order of up to 20 mm, and smaller wall thicknesses, for example in the range from 1 to 5 mm.
FR 800 531 further describes a device and a method for thermally pre-stressing large glass objects, wherein relative rotation can be produced between the glass object and the blowing device.
GB 512,976 discloses a device and a method for thermally pre-stressing large glass tubes, wherein relative rotation can be produced between the glass object and the blowing device. Cooling air is blown into an axial end of the object. Also cooling air is blown out of a plurality of nozzles arranged at equal spacings from each other along a tube that extends parallel to the glass tube to be treated.
These publications, however, do not teach how very small glass objects can be thermally pre-stressed. Since small objects tend to have a small heat capacity, it is very difficult to thermally pre-stress such objects, which cool very rapidly thereby counteracting the thermal pre-stressing operation.